A roller cone earth-boring drill bit has a number of cones, typically three, each mounted rotatably to a bearing pin. Each cone rotates about its axis when the bit body rotates around the bit axis. The cones have rows of cutting elements, which may be teeth integrally formed in the cone metal, or tungsten carbide inserts pressed into mating holes in the cone metal.
Each cone will have an outermost or heel row near a gage surface of the cone and one or more inner rows. One or more of the cones will have cutting elements located near or on the nose of the cone. In some cases the inserts in the adjacent row closest to the heel row will be staggered or alternate with the inserts in the heel row.
The inner rows of each cone are arranged at different distances from the bit axis for cutting different portions of the borehole bottom. Normally, at least two of the cones will have heel rows that are located at substantially the same distance from the bit axis. Some of the adjacent rows may be approximately the same distance from the bit axis. When all three cones are rotated into a single section plane, these heel row inserts and some of the adjacent row inserts will superimpose or overlap at least partially on one another. The inner rows are normally spaced at different distances from the bit axis to cover the remaining portions of the borehole bottom.
When rows of inserts of different cones overlap each other, tracking can result. That is the inserts of the two or more cones in those rows tend to fall into the same holes in the borehole bottom, building up ridges on the bottom. These ridges are detrimental because they can contact the supporting metal of the cone, lower the load on the inserts, and cause wear.
In the prior art, steps are taken to reduce tracking. Usually, a bit designer tries to provide at least one of the heel rows with the maximum number of inserts because these rows engage more of the borehole bottom than any other rows. The maximum number is limited by the requirement of adequate supporting metal in the cone body. A typical approach to further reduce tracking is to increase the pitch in the overlapping heel row of another cone. The wider pitch, or distance between center lines of inserts, tends to break up the ridges that form between the impressions made by the more closely spaced heel row inserts. In addition, the adjacent row inserts are staggered with the wider pitch heel row. While workable, a greater pitch means fewer inserts in the adjacent row. This reduces the durability of the adjacent row and can result in even higher ridge build-up between the adjacent row inserts.